[workflow] Call `scanWorkflow` concurrently
[workflow] Call `scanWorkflow` concurrently

`scanWorkflow` involves a remote call to a tablet to get the
`copy_state` rows for the workflow stream associated with that tablet.
Calling this concurrently gives us a noticeable speedup, especially on
workflows involving many streams.

Signed-off-by: Andrew Mason <8a240ff75aa1467ba5eba4db0c8200b61262547e@slack-corp.com>

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